1. Field of the Invention
The present invention relates to bits for drilling subterranean formations, and more specifically, to rolling cutter bits (also termed "tri-cone" or "rock" bits) and superabrasive-tipped, insert-type cutting elements for use on the cutters of such bits.
2. State of the Art
The development of rotary drilling techniques facilitated the discovery and development of deep oil and gas reserves, first in the United States and subsequently throughout the world. The rolling-cutter (also sometimes called "rolling cone" herein) rock bit was a significant advance in drilling techniques, as only softer, more shallow formations could previously be drilled on a commercially-viable basis with early cable-tool equipment and primitive, metal-cutter drag bits. The rolling-cone bit invented by Howard R. Hughes, disclosed in U.S. Pat. No. 939,759, was capable of drilling the hard caprock at the now-famous Spindletop field near Beaumont Tex., thus revolutionizing oil and gas drilling.
Today's rolling-cone or rolling-cutter bits drill at much-improved penetration rates and for vastly greater durations over varying formation intervals in comparison to the original Hughes bit, due to improvements in designs and materials over many intervening decades. However, the basic principles of drilling with rolling-cutter bits remain the same, although are understood to a far greater extent than when this type of bit was originally developed.
Rolling-cone earth boring bits generally employ cutting elements on the cones or cutters to induce high contact stresses in the formation being drilled as the cutters roll over the bottom of the borehole during a drilling operation. These stresses cause the rock of the formation being drilled to fail, resulting in disintegration and penetration of the formation. The cutters of the bit usually, in the context of conventional bit design, rotate or roll about axes which are inclined with respect to the geometric or rotational axis of the bit itself, as driven by the drill string. The rotational axes of the rolling cutters are, in fact, disposed at a substantial angle to the bit axis, extending downwardly and inwardly from the bit leg adjacent the outer bit perimeter toward the centerline of the bit, and the conical shape of most conventional cutters is matched to the cutter axes to cause a plurality of integral teeth or press-fit inserts (generally "cutting elements") projecting outwardly from the side exterior of the cutter to engage the formation along lines of contact extending from the outer base or heel surface of each cutter shell inwardly toward the centerline of the bit. Typically, the cutting elements are arranged in multiple, substantially parallel, generally circumferential rows about the exterior of the cutter, although spiral and other cutting element arrangements are also known in the art. Cutting elements are also located about the bottom periphery of the cutter cones, commonly called the gage surface, and additional cutting elements or scraping elements may be disposed along the intersection of the gage surface and the heel surface of the cutter.
Due to the bit design as briefly described above, and also due to variations in formation material as well as weight on bit (WOB), torque and rotational speed as transmitted to the bit through the drill string, a cutter does not necessarily just roll or rotate over the bottom of the borehole with little or no relative movement between the cutting elements and the formation, but also slides against the formation material due to offset of the cutter axis from a radial plane and variations from a true rolling, perfectly conical cutter geometry. Such sliding also may be caused by precession of the bit about its centerline. Further, the incidence of sliding may be of particular significance during directional drilling operations, wherein the bit is being oriented to drill a path which is not absolutely coincident with its centerline due to the influence of eccentric stabilizers, bent subs, bent housings, or other passive or fixed steering elements, or by active steering mechanisms (arms, pads, adjustable stabilizers, etc.) included in the bottomhole assembly. Such sliding causes the cutting elements of the bit to gouge or scrape the formation, providing another, albeit unintended, mode of cutting in addition to the aforementioned crushing mode.
A generic term for the gouging or scraping action of sliding cutting elements removing formation material is "shear-type cutting", which is the primary mode of cutting in so-called fixed-cutter or "drag" bits, wherein non-movable cutting elements, often having cutting tables or projecting teeth comprised of highly wear-resistant superabrasive materials, cut chips or even elongated strips of material from the formation being drilled. However, the existence of a shear-type cutting in rolling-cutter bits, while recognized, has not been extensively developed in the art. U.S. Pat. Nos. 5,282,512; 5,341,890; and 5,592,995, as well as copending U.S. patent application Ser. No. 08/695,509, the latter filed Aug. 12, 1996 and assigned to the assignee of the present invention, each disclose cutting elements including design features for cutting in shear for use on rolling-cone cutter. Each of the aforementioned patents and application also discloses the use of a discrete, relatively small, preformed diamond element carried on, or in a cavity or recess in, the exterior of the metal insert, typically of a carbide such as cemented tungsten carbide (WC). The metal insert portions of these cutting elements provide a large majority of the exterior surface of the inserts exposed to the formation, drilling fluid, and formation debris.
Another approach to forming insert-type superabrasive cutting elements has been to form a jacket or coating of superabrasive (diamond) material over an insert body of WC, although other metals and alloys have been employed in the art. U.S. Pat. Nos. 4,604,106; 5,045,092; 5,145,245; 5,161,627; 5,304,342; 5,335,738; 5,379,854; 5,544,713 and 5,499,688, as well as copending U.S. patent application Ser. No. 08633,983, filed Apr. 17, 1996, disclose such jacketed or coated inserts. Also disclosed in some of these patents is the use of discrete, relatively small diamond elements placed or formed in recesses in the surface of an insert, such elements either being exposed to the insert exterior, or covered by a diamond jacket or coating. U.S. Pat. No. 4,109,737 discloses the use of a thin polycrystalline diamond compact layer on the end of a stud-type cutting element for use in drag bits.
Yet another approach to a superabrasive insert-type cutting element for rolling cutter bits is disclosed in U.S. Pat. Nos. 5,159,857; 5,173,090; and 5,248,006. These patents take a radically different approach to superabrasive inserts, using a high-pressure, high-temperature formed, polycrystalline diamond compact core surrounded by a relatively thin, tubular, hard metal jacket and in some cases, an integral base or floor of the same metal, forming a cup-like, diamond-filled structure. The metal jacket is initially formed with an excess wall thickness so that the insert can be machined to a desired diameter for insertion in a rolling cutter.
In an insert comprised primarily of metal and having only small, discrete diamond elements placed thereon at one or two select locations, precise predictions of magnitudes and orientations of cutter and insert loading are required to ensure correct placement and orientation of the diamond elements. Further, the metal insert body and discrete diamond elements in some instances are separately preformed, and require subsequent mutal attachment by brazing or other metallurgical bonding techniques.
In an insert having only a superabrasive (diamond) jacket, the underlying metal stud material ultimately supports the loading to which the insert is subjected during drilling, whether it be the compressive-type loading for which inserts are primarily designed, or the shear-type loading previously mentioned above. The diamond jacket may itself thus be stressed in tension, under which it is very weak and exhibits a remarkably low strain to failure ratio, due to yielding of the underlying metal stud. The yielding of the stud material may result in cracking, spalling, fracture or delamination of the diamond jacket from the stud. Approached from another standpoint, the stress gradient in a thin diamond jacket or shell is extremely great; leading to early failure if not supported by an equally unyielding material. Thermal stresses may also aggravate the aforementioned problems. Further, shear forces may also stress the diamond/metal interface (already residually-stressed from fabrication) in tension, again causing degradation of the diamond jacket or its bond to the stud.
Diamond-core inserts having only a metal shell or jacket surrounding the diamond mass extending substantially the length of the insert do not suffer from loading-induced damage in the same manner as the diamondjacketed inserts since the diamond core material itself takes the loading, but such inserts cannot normally sustain impact or high stress on the superabrasive tip without cracking of the metal shell or jacket, which frequently leads to loss of the insert from the cutter. Moreover, the diamond-core inserts require a relatively large volume of expensive diamond particles for forming the diamond core, and the method of forming such diamond-core cutting elements yields a very small number of parts for each run of the diamond press.
It has been contemplated to form a drag bit diamond cutting element with a substantial superabrasive structure, as disclosed in commonly-assigned copending U.S. patent application Ser. No. 08/602,076 and U.S. patent application Ser. No. 08/602,050, each filed on Feb. 15, 1996 and hereby incorporated herein by this reference. However, only somewhat generalized developments were disclosed regarding the concept of forming rolling-cutter inserts in terms of specific internal and external structure, or to with regard the mounting inserts in the rolling cutter itself.
Thus, there remains a need for an effective, robust, insert-type superabrasive cutting elements having utility on rolling cutter type bits, susceptible to fabrication in an efficient and economical manner using known manufacturing techniques and mountable to a rolling cutter in a manner which minimizes potential loss of, or damage to, the cutting element during service.